1. Field of the Invention
The invention relates to dual fuel engines and, more particularly, relates to a non-invasive system for controlling the supply of pilot fuel to a dual fuel engine by governor control and to a method of using such a system.
2. Discussion of the Related Art
Recent years have seen an increased demand for the use of gaseous fuels as a primary fuel source in compression ignition engines. Gaseous fuels such as propane or natural gas are considered by many to be superior to diesel fuel and the like because gaseous fuels are generally less expensive, provide equal or greater power with equal or better mileage, and produce significantly lower emissions. This last benefit renders gaseous fuels particularly attractive because recently enacted and pending worldwide regulations may tend to prohibit the use of diesel fuel in many engines. The attractiveness of gaseous fuels is further enhanced by the fact that existing compression ignition engine designs can be readily adapted to burn gaseous fuels.
One drawback of gaseous fuels is that they exhibit significantly higher autoignition temperatures than do diesel fuel, oil, and other liquid fuels traditionally used in compression ignition engines. Accordingly, the temperature of the gaseous fuels does not increase sufficiently during operation of standard compression ignition engines for self-ignition. This problem is overcome by injecting a small charge of a pilot fuel, typically diesel fuel, into the combustion cylinders of the engine in the presence of a charge of a compressed gaseous fuel/air mixture. The pilot fuel is distributed throughout the gas/air mixture, ignites upon injection and subsequent compression, and bums at a high enough temperature to ignite the gaseous fuel charge.
The cost of convening a pre-assembled compression ignition engine to a dual fuel engine or of assembling a new dual fuel engine can be minimized by employing the stock diesel fuel injection components of the engine as a pilot fuel supply system and by merely adding a gaseous fuel supply system to the otherwise unmodified diesel engine. In the case of engines employing rack-controlled high pressure in-line pumps for the supply of diesel fuel to liquid fuel injectors, this conversion has traditionally entailed the removal of the stock mechanical or electronic governor and the direct control of the rack using a pneumatic or hydraulic actuator. The actuator is controlled, based upon sensed engine operating parameters, to position the rack so as to deliver diesel pilot fuel to the cylinders at a quantity per stroke demanded by the instantaneous engine operating conditions. This technique, while effective, exhibits marked drawbacks and disadvantages.
First, it is relatively complex and invasive. The existing governor must be removed and the existing pump disassembled and modified to provide access to the rack by the actuator. In the case of a retrofit operation, this conversion may necessitate the removal of the entire engine from the associated vehicle and render retro-fitting cost prohibitive. Second, elimination of an existing governor necessarily eliminates the inherent safety features of the governor, namely, the setting of minimum and maximum fuel delivery quantities per stroke independently of governor actuating lever position.